My son is 12 and on the cusp of puberty. For the last six months, his behaviour towards me and his 15-year-old sister has become oversexualised and inappropriate. This has included making lewd remarks and suggestions to her. He often grabs her, or strokes her hair or arms. He does the same to me, using language that sounds like lyrics from suggestive love songs. When going to and from the bathroom, he exposes himself and makes lewd remarks.

We’ve made it clear we don’t like it and want him to stop. He laughs and says he didn’t mean it. He rarely behaves like this in front of his father (we all live together). He goes to an all-boys school and I haven’t had reports of this there.

I’m at the end of my tether. I want to show him, in front of his sister, that his behaviour could be classed as criminal. I’ve tried punishments that we use for other poor behaviour. Sometimes this stops him temporarily. In general, he is quite an anxious, angry and unhappy person at home. I monitor his internet access and I haven’t found evidence he watches porn or adult content. He mostly uses it for gaming.

Teenagers often test the boundaries with their parents, but it’s not usual for boys to make suggestive sexual comments to female members of the family, and less usual still that they expose themselves. This is the age they tend to become more self-conscious and inhibited – so something is clearly going on with your son and you need to find out what.

We both wondered what your husband’s reaction to your son’s behaviour was – whether or not he witnesses it himself? He needs to be more involved than he seems to be. It’s important for men to call out inappropriate behaviour in other men, and that starts in the home.

Music said that, often, if children are experiencing something they cannot deal with, they will seek to make others feel what they are feeling. It’s as if they are throwing it out there to say, “This is what I’m dealing with.” So the child who feels shame may seek to make others ashamed, the child who feels left out rejects others, and so on. “You don’t often act out so overtly unless you’ve been exposed to something that’s been overwhelming,” explains Music.

You may not have found porn or adult content on his computer, but that does not mean he hasn’t been exposed to it. It is likely he will have seen something. He could have seen or heard something via gaming; get more involved (you and your husband) in what he does online, which is best done by showing an interest rather than hectoring.

The fact he’s angry and anxious worries me further. Was he always like this? Music asks: “How did your son deal with unhappiness as a child?”

I would also talk to the school to get a better picture of what’s going on there. “Is he being bullied, and is he bullying you and his sister in turn?” asked Music. Has he recently changed schools?

Music was also interested in the dynamics of your house: “Who else lives there, what else is going on there, what are the power dynamics between you and your husband?”

There was scant information in your letter and little curiosity about why your son is doing this. The key to stopping it is to understand why he’s doing it. “It could be sexual urges,” said Music, “or it could be he’s using the sexual as a language to enact something else, like asserting power.” How do you generally deal with feelings in the family? Are they allowed or are they buried? Your son might have noticed and enjoyed getting a reaction out of you. But, again, you need to look at why he feels this compulsion.

In the meantime, his behaviour must be addressed and your daughter told she can react to protect herself. “Boundaries and authority are essential,” says Music, “But you and your husband need to set those up together. Try to stop his behaviour and then work out why it’s happening. Your son needs to understand there are consequences, but you need to make space to think about why he’s doing this.”